1. Field of the Invention
The invention relates to an eyepiece and a finder optical system, and in particular, to a finder for a single-lens reflex camera suitable for a camera that has a frame size of about a half of the diagonal length of a so-called 35 mm silver-halide film, and a digital single-lens reflex camera.
2. Description of Related Art
Recently, special attention has been devoted to digital cameras as an alternative to so-called 35 mm silver-halide film cameras. Sizes of CCDs used as image sensors of such digital cameras usually range from a fraction to a few tenths in diagonal length, compared with those of 35 mm silver-halide films. When the eyepiece of a conventional silver-halide film camera is used as it is in a camera that has such a small frame size, an observation image is reduced in size. Thus, in the case where a single-lens reflex system is introduced into the camera of this type, it is necessary to increase the magnification of a finder in order to facilitate image viewing.
Here, in order to increase the magnification of the finder, the focal length of the eyepiece must be reduced. In general, however, when the focal length of the eyepiece is reduced, a distance from a surface to be observed (a real image of an object) to the eyepiece (this distance is hereinafter referred to as a back focus) becomes short.
On the other hand, in order to provide a finder that is easy to see, it is necessary that a distance from the eyepiece to an observer's pupil, namely, a so-called eyepoint distance, is long and eclipse is practically eliminated. Generally, in order to ensure a long eyepoint distance, there is the need to increase the diameter of the eyepiece and to enlarge an effective aperture of the exit surface of an image inverting optical member. As a result of oversizing of the image inverting optical member, a developed optical path length in a prism or a mirror is increased. That is, a long back focus is required to obtain a high eyepoint.
As mentioned above, a high finder magnification is obtained and at the same time, various optical members such as prisms and mirrors can be arranged. In order to realize a high-eyepoint eyepiece, it is essential to ensure a long back focus, in spite of a short focal length.
Prior art examples of eyepieces where the back focus is longer than the focal length are disclosed in References 1-3 shown below.                Reference 1: Japanese Patent Publication No. Hei 7-107581        Reference 2: Japanese Patent No. 2726261        Reference 3: Japanese Patent Kokai No. Hei 2-304409        
A finder optical system for common single-lens reflex cameras is fundamentally designed so that a light beam is reflected upward by a main mirror, for example, a quick-return mirror, provided on the optical path of a photographing optical system; a primary imaging plane for forming an object image turned left to right and inverted is provided; and the image is turned through prisms or mirrors into an erect image. The quick-return mirror is removed outside a photographing optical path in photography, and the light beam of the object image is conducted to an imaging surface so that photography is performed.
Techniques that the finder optical systems for conventional single-lens reflex cameras are fundamentally designed are disclosed, for example, in References 4-7 described below.                Reference 4: Japanese Patent Kokai No. Hei 01-185622        Reference 5: Japanese Patent Kokai No. Hei 10-197914        Reference 6: Japanese Patent Kokai No. 2003-307764        Reference 7: Japanese Patent Kokai No. 2001-8066        
Reference 4 proposes the technique that an image formed in a Porro prism located behind a quick-return mirror by light transmitted through the quick-return mirror provided on the optical path of a photographing optical system is optically equivalent to an object image turned left to right and inverted that is formed on an imaging surface by light reflected by the quick-return mirror.
Reference 5 proposes the technique that after light is reflected upward by the quick-return mirror provided on the optical path of the photographing optical system, an erect image formed by turning an image by the Porro prism is equivalent to an image formed by light transmitted through the quick-return mirror.
Reference 6 proposes the technique that light is reflected upward by the quick-return mirror provided on the optical path of the photographing optical system, and after being reflected by a pellicle mirror, is further reflected by a roof prism or a roof mirror, and thereby an image to be observed is equivalent to an image formed on the imaging surface by light transmitted through the quick-return mirror.
Reference 7 proposes the technique that light is reflected upward by a first beam splitter provided on the optical path of the photographing optical system and is further reflected by a second beam splitter provided in the upper portion, and an object image thus obtained through an eyepiece is equivalent to an image formed by light transmitted through the first beam splitter.
On the other hand, where the finder optical system, such as that of the single-lens reflex camera set forth in each of References 4-6 or that of the digital single-lens reflex camera set forth in Reference 7, is fundamentally designed, an image turned by the prism is formed as the erect image, and thus light is once reflected toward the object side to form the erect image.
When AE (automatic exposure) and an optical system such as an imaging optical system are provided, it is necessary to ensure sufficient space for them.
The digital single-lens reflex camera that uses a large-sized CCD of 4/3 in., APS size, or 35 mm full size, mainly adopted in this type of camera, fails to have a through image function (the function of observing an object through an electronic finder in an LCD) that is common in a compact digital camera.